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Henselian ring
In mathematics, a Henselian ring (or Hensel ring) is a local ring in which Hensel's lemma holds. They were introduced by Azumaya (1951), who named them after Kurt Hensel. Azumaya originally allowed Henselian rings to be non-commutative, but most authors now restrict them to be commutative.
Some standard references for Hensel rings are (Nagata 1962, Chapter VII), (Raynaud 1970), and (Grothendieck 1967, Chapter 18).
Definitions
In this article rings will be assumed to be commutative, though there is also a theory of non-commutative Henselian rings.
A local ring R with maximal ideal m is called Henselian if Hensel's lemma holds. This means that if P is a monic polynomial in R[x], then any factorization of its image P in (R/m)[x] into a product of coprime monic polynomials can be lifted to a factorization in R[x].
A local ring is Henselian if and only if every finite ring extension is a product of local rings.
A Henselian local ring is called strictly Henselian if its residue field is separably closed.
A field with valuation is said to be Henselian if its valuation ring is Henselian.
A ring is called Henselian if it is a direct product of a finite number of Henselian local rings.
Henselian rings in algebraic geometry
Henselian rings are the local rings of "points" with respect to the Nisnevich topology, so the spectra of these rings do not admit non-trivial connected coverings with respect to the Nisnevich topology. Likewise strict Henselian rings are the local rings of geometric points in the étale topology.
Henselization
For any local ring A there is a universal Henselian ring B generated by A, called the Henselization of A, introduced by Nagata (1953), such that any local homomorphism from A to a Henselian ring can be extended uniquely to B. The Henselization of A is unique up to unique isomorphism. The Henselization of A is an algebraic substitute for the completion of A. The Henselization of A has the same completion and residue field as A and is a flat module over A. If A is Noetherian, reduced, normal, regular, or excellent then so is its Henselization.
Similarly there is a strictly Henselian ring generated by A, called the strict Henselization of A. The strict Henselization is not quite universal: it is unique, but only up to non-unique isomorphism. More precisely it depends on the choice of a separable algebraic closure of the residue field of A, and automorphisms of this separable algebraic closure correspond to automorphisms of the corresponding strict Henselization.
Example. The Henselization of the ring of polynomials k[x,y,...] localized at the point (0,0,...) is the ring of algebraic formal power series (the formal power series satisfying an algebraic equation). This can be thought of as the "algebraic" part of the completion.
Example A strict Henselization of the field of p-adic numbers is given by the maximal unramified extension, generated by all roots of unity of order prime to p. It is not "universal" as it has non-trivial automorphisms.
Examples
Every field is a Henselian local ring.
Complete hausdorff local rings, such as the ring of p-adic integers and rings of formal power series over a field, are Henselian.
The rings of convergent power series over the real or complex numbers are Henselian.
Rings of algebraic power series over a field are Henselian.
A local ring that is integral over a Henselian ring is Henselian.
The Henselization of a local ring is a Henselian local ring.
Every quotient of a Henselian ring is Henselian.
A ring A is Henselian if and only if the associated reduced ring Ared is Henselian (this is the quotient of A by the ideal of nilpotent elements).
If A has only one prime ideal then it is Henselian since Ared is a field.
References
Azumaya, Gorô (1951), "On maximally central algebras.", Nagoya Mathematical Journal 2: 119–150, ISSN 0027-7630, MR 0040287
Danilov, V. I. (2001), "Hensel ring", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
Grothendieck, Alexandre (1967), "Éléments de géométrie algébrique (rédigés avec la collaboration de Jean Dieudonné) : IV. Étude locale des schémas et des morphismes de schémas, Quatrième partie", Publications Mathématiques de l'IHÉS 32: 5–361, doi:10.1007/BF02732123
Kurke, H.; Pfister, G.; Roczen, M. (1975), Henselsche Ringe und algebraische Geometrie, Mathematische Monographien II, Berlin: VEB Deutscher Verlag der Wissenschaften, MR 0491694
Nagata, Masayoshi (1953), "On the theory of Henselian rings", Nagoya Mathematical Journal 5: 45–57, ISSN 0027-7630, MR 0051821
Nagata, Masayoshi (1954), "On the theory of Henselian rings. II", Nagoya Mathematical Journal 7: 1–19, ISSN 0027-7630, MR 0067865
Nagata, Masayoshi (1959), "On the theory of Henselian rings. III", Memoirs of the College of Science, University of Kyoto. Series A: Mathematics 32: 93–101, MR 0109835
Nagata, Masayoshi (1975) [1962], Local rings, Interscience Tracts in Pure and Applied Mathematics 13 (reprint ed.), New York-London: Interscience Publishers a division of John Wiley & Sons, pp. xiii+234, ISBN 978-0-88275-228-0, MR 0155856
Raynaud, Michel (1970), Anneaux locaux henséliens, Lecture Notes in Mathematics 169, Berlin-New York: Springer-Verlag, pp. v+129, doi:10.1007/BFb0069571, ISBN 978-3-540-05283-8, MR 0277519
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